Increased flux of Na and K ions across the cell membrane with a resultant increase in intracellular K ion concentration has been proposed to play a critical role in the regulation of cell proliferation (Rozengurt and Heppel, 1975). The present study will demonstrate if this increased ion flux is a necessary prerequisite for replication of Balb/c-3T3 cells. In addition, regulation of ion flux by the cell growth stimulating factors from serum, platelet-derived growth factor (PDGF), and somatomedin, as well as by the tumor promotor tetradecanyol phorbol acetate (TPA) and by the oncogenic virus SV40 will be explored. The effects of these agents on ion flux will be studied both in whole cells and in membrane preparations and membrane vesicles from cells arrested at several defined "points" in the G0/Gl phase of the cell cycle. Systematic analysis of direct effects of these agents on ion flux in membrane vesicles from the growth-arrested cells will define the roles of Na-K ions ATPase and other ion transport mechanisms in the regulation of cell growth.